1. Field of the Invention
The present invention relates to a piezoelectric actuator applicable to a fuel-injection apparatus of direct gasoline-injection engines, diesel engines or the like and having piezoelectric elements that, when applied with an electric current, may exhibit strains in dimension thereby driving an object. The present invention further relates to a fuel-injection apparatus provided with the same actuator.
2. Description of the Prior Art
Of the various fuel-injection apparatus that have been conventionally developed, for example, the pressure-accumulated, fuel-injection apparatus is widely used, in which fuel stored in the common rail under pressure is injected into the combustion chambers by the closure and open of the valves built in the injectors.
An exemplary pressure-accumulated, fuel-injection apparatus is disclosed in Japanese Patent Laid-Open No. 77924/1998, which is comprised of a body having fuel-discharge orifice, a needle valve movable in a space in the body in a reciprocating manner so as to open and close at its one axial end the fuel-discharge orifices, a balance chamber in which the needle valve is exposed at its axially opposite end serving as a pressure-supporting face to control the amount of lift of the needle valve, a fuel-supply passage for applying the fuel pressure to the balance chamber, a fuel-discharge passage for relief of the fuel pressure in the balance chamber, a valve for opening and closing the fuel-discharge passage, and a piezoelectric actuator for operating the valve. The actuator-operated valve is composed of a valve stem extending through the fuel-discharge passage into the balance chamber, and a tapered valve head provided at the end of the valve stem and having a valve face to make a contact with a valve seat formed at the ingress of the fuel-discharge passage.
The piezoelectric actuator may be driven with the command pulse width and the timing of "turned on", depending on the amount of fuel to be injected per cycle and the timing of fuel injection, which are defined in accordance with the engine operating conditions such as the engine rpm, engine load, accelerator position and the like. When the command pulse is made to fall for a duration during which the injectors are to be operated, the piezoelectric elements is applied with a preselected voltage for the duration, thereby kept in conductive phase. The electric current flowing through the piezoelectric elements causes the elements any strains in dimension to thereby vary the distance between the piezoelectric elements of laminated structure with the result of varying the total stroke of the laminated elements. This variation in total stroke comes into action on the valve stem of the actuator-operated valve, which is in turn moved along its axial direction.
The instant the piezoelectric actuator is energized, the valve face of the tapered valve head moves off its seat, whereby the actuator-operated valve opens the fuel-discharge passage, resulting in relieving the fuel pressure in the balance chamber to the fuel-discharge passage. Consequently, the fuel pressure acting on the pressure-support surface of the needle valve is reduced to allow the needle valve to lift so that the fuel may be injected out of the fuel-discharge orifices into the combustion chambers. In contrast, on deenergizing the piezoelectric actuator, the tapered valve head moves, together with the valve stem extending through the fuel-discharge passage into the balance chamber, towards the egress of the fuel-discharge passage and thus the valve face is pressed against the valve seat to block the fuel-discharge passage. At this event, the fuel pressure in the balance chamber increases so that the needle valve closes the fuel-discharge orifices to stop the fuel injection out of the discharge orifices in the injectors into the combustion chambers.
In the meantime, since the piezoelectric actuator is superior in responsive property of occurrence and disappearance of strains to the energization and deenergization thereof, the piezoelectric actuator for the valve control of the fuel-injection apparatus has the advantage of achieving the rapid operation of the start and stop of the fuel injection with less response lag even the fuel injection cycle is very short in period in compliance with the high-speed rotation of the engines.
In the prior piezoelectric actuators, nevertheless, the instant the command pulse starts to fall or the injector is turned on, the piezoelectric elements are suddenly excited under an electric voltage that increases steeply. For the reason above, the prior actuator has trended to be driven over its design stroke at the initial timing of its driving, which might cause the piezoelectric actuator itself to oscillate, thereby obstructing the actuator from consistency with the desired stroke. The over stroke is a major cause of the damage in the actuator.
The problems in the prior piezoelectric actuator as described above will be explained with reference to FIGS. 6 to 8.
Now assuming that an exciting pulse is applied to raise up a command pulse after a lapse of 1 ms from a timing at which the command pulse falls as shown with a solid line in FIG. 6, the voltage applied across the piezoelectric elements, as shown with a solid line in FIG. 7, causes an overshoot, refer to a circle A, just after the command pulse has started to fall, and thereafter starts to oscillate with a resonance amplitude of about 20V. As apparent from the above, the exciting pulse in the prior art causes a resonance frequency in the voltage applied across the piezoelectric elements, which is defined by a design constant of the piezoelectric elements. Moreover, as shown with a solid line in FIG. 8, an overshoot, refer to a circle B, happens in a dimensional strain or displacement in the piezoelectric elements and also a mechanical oscillation having a resonance amplitude of about 30 .mu.m, or a ringing, occurs in the piezoelectric elements. On exciting the piezoelectric elements, as explained above, the resonance takes place owing to the ringing. Thus, the resonance in the piezoelectric elements becomes obstructive to the desired displacement and causes the dielectric breakdown across the terminals of the piezoelectric elements. Moreover, the displacement in the piezoelectric elements does not damp immediately, but continues to oscillate even after the command pulse has raised up, or the exciting pulse is turned off.
Consequently, the prior piezoelectric actuator has for its technical subject to eliminate the overshoot in the voltage across the piezoelectric elements and in the displacement in the piezoelectric elements, and to prevent the ringing thereby preventing the occurrence of resonance phenomenon.